This story was first published wired Reprinted here as part of. climate desk collaboration.
Mexico City may not look soft at first glance due to its extensive buildings and concrete, but it actually looks soft. Ever since the Spanish conquistadors drained Lake Texcoco for further urbanization, the land has been gradually compressed under weight. This is a phenomenon known as land subsidence, and the consequences are dire.mexico city is Up to 20 inches of subsidence per yearwreaking havoc on infrastructure.
This includes the city's subway system, which is the second largest in North America after New York City. Now, scientists can use satellites to closely measure the rate of subsidence across Mexico City and map where subsidence could damage railways. „Here in the city, you get used to buildings leaning a little bit,“ says Dario Solano Rojas, a remote sensing scientist at the National Autonomous University of Mexico. “I can feel the rails wobbling. Riding the Mexico City subway feels weird. I don’t know if it’s dangerous or not — you feel I think it's dangerous, but I'm not so sure. ”
recently study In the journal Scientific Reports, Solano-Rojas sought certainty. He and his team used radar satellite data to measure how elevation changed across the city between 2011 and 2020. Subsidence is not uniform. Rates are determined by several factors. The most dramatic example worldwide is due to over-extraction of groundwater. Pump out enough liquid and the ground will collapse like an empty water bottle.This is why Jakarta, Indonesia is sinking Up to 10 inches per year.Land has sunk in California's San Joaquin Valley 28 feet long For the past century, it was caused by farmers pumping too much groundwater.
A similar aquifer drain is occurring in Mexico City. facing a worsening water crisis. „The underground is like a sponge. When you take water out of it, it loses volume and deforms,“ Solano-Rojas says. How much volume it takes depends on the sediments underlying a particular part of the city. Ancient lakes were not neatly layered with equal proportions of clay and sand in all areas. “That causes different movements on the ground,” Solano-Rojas adds.
Land subsidence rates throughout Mexico City vary widely, from 20 inches per year to none at all. Mexico City is built on solid volcanic rock. This results in „unequal settlement,“ where the land sinks not only differently from square mile to square mile or block to block, but from square foot to square foot. Roads, railways, and buildings that sink differently from one end to the other become unstable.
The sloped road traffic barrier at Akatitla station, shown above, is constructed in this way. And below that is the deformation of the tracks at Oceania Station. If the land is sinking at a uniform rate in any of these locations, the problem may not occur because the railroad tracks and roads will also sink uniformly. “We found that some sections of the subway system were moving faster than they were designed,” Solano-Rojas said. The study found that nearly half of the subway's elevated sections were experiencing uneven settlement. This is the normal threshold for a system to require service before the end of 50 years, at which point rehabilitation or repair of the segment will be required to continue optimal operation. means.
Sistema de Transporte Colectivo, which operates Mexico City's metro system, did not provide comment on the matter despite repeated inquiries.
The subway system is, by its very nature, a vast network of routes. Mexico City includes: 140 miles Tracks run underground under the subway, above ground as seen above, and on elevated platforms. „We're going from areas that are really stable to areas that are sinking 30 centimeters a year or even almost 40 centimeters a year,“ Solano-Rojas said. „So the goal here was to see where the most damage could occur.“
That damage comes in several forms. As the land sinks, ditches form and collect rainwater, which can lead to flooding along railway lines. That could disrupt the electrical system that powers the trains, Solano-Rojas said.
Elevation changes can also increase the slope of the rail. Solano-Rojas said subway trains are designed to operate on slopes of up to 3.5 percent, but some sections of the track are currently at double that due to subsidence. „Small changes in the railroad's level can easily cause a train to derail,“ says Richards, an environmental security expert at Virginia Tech who studies land subsidence and was involved in the new paper. Manucher Sirzai says that he has not done so. “Most infrastructure has certain thresholds. Some difference in subsidence is allowed. But in many cases, rates like those seen in Mexico City, for example, are not taken into account.”
Solano-Rojas and his colleagues found subsidence in the area of an overpass near Olivos station, which collapsed in 2021 while a subway train was passing through it. “When we did some of this analysis before 2021, we detected that the displacements were different in the region,” he says Solano-Rojas. „We were like, 'Oh, yeah, maybe something could happen here in the future.'“ I don't think it was a coincidence that we found this. ” Solano-Rojas cautioned that the potential contribution of land subsidence to the disaster requires further evaluation, and that official investigations are pending. Point out construction mistakes And no mention of subsidence.
In this study, the researchers looked at above-ground subway infrastructure rather than subway segments. Basically, it's the part of the system that you can visually check. (The photo below shows the differential settlement of columns supporting an overpass.) However, providing system operators with information about how quickly the infrastructure is sinking can help their work become more responsive to intervention. We hope that you will be able to provide information. Engineers can add material under railroad tracks to restore lost elevation, for example. But strengthening the subway could be even more difficult. „There's no concrete solution to that,“ Shirzaei said. “Most of the time, when something like that happens, we just end up canceling the project and trying to open new lanes.”
This is not just a Mexico City problem. Earlier this year, Shirzaei and his colleagues East Coast infrastructure faces serious challenges Because of the slow but steady subsidence. They calculate that her 29,000 square miles of Atlantic coastline are exposed to up to 0.08 inches of subsidence per year, affecting up to 14 million people and 6 million properties. Approximately 1,400 square miles are sinking by up to 0.20 inches per year.
Researchers have found that differential subsidence threatens not only railways, but all kinds of critical infrastructure, including levees and airports. Large cities like New York City have the added problem of enormous weight. push down on the ground, which alone leads to subsidence. Bay area too sinking.Subsidence is occurring on both coasts. significantly worsen the problem of sea level rise: Just as the water rises, the land falls.
No matter where in the world it is happening, people must stop over-extracting groundwater to slow land subsidence. The new system is already reducing pressure on the aquifer.It's getting cheaper and cheaper Recycle toilet water into drinking water, for example.and many more cities Deploying a “sponge” infrastructure — There are lots of green spaces where rainwater flows. percolate into the underlying aquifer, basically re-expanding the land to prevent land subsidence. These efforts are becoming increasingly urgent as climate change worsens droughts and puts ever greater pressure on groundwater supplies in Mexico City and other parts of the world.
Increased satellite data will allow cities to better address land subsidence that cannot be avoided anytime soon. „I really feel that the government has an opportunity to use this type of research to develop a more structured plan of action,“ Solano-Rojas says.